When optical systems use high power lamp light sources, there can be a significant radiative proportion of the light emitted in the UV region. When organic materials are in the light path there will be a degradation of this material over time. This degradation effect has also been recognized for such applications as LCD projection systems, and for high power laser pump lamps.
It is well known that UV radiation can also cause degradation and discoloration in such items as paints, fabrics and plastics. Specifically, electromagnetic energy in the ultraviolet spectrum (i.e., between 4 and 400 manometers), causes paints to fade, causes rubber to crack, and plastics to crumble with time. Therefore, strong UV absorption by architectural glazing materials is beneficial.
The sun is not the only light source that emits UV. Various artificial lighting sources like Hg or Xe ARC and halogen lamps emit UV radiation. UV absorbing glasses can be used that block the entire range of the UV emission of these sources. However, as a result of this absorption, with prolonged usage these glasses tend to solarize or darken with time, especially from the absorption of the shorter wavelength, higher energy portion of the UV region. Accordingly, there is an interest in absorbing filters that show minimal loss of visible transmission.
It is also common knowledge that photochromic glasses are activated by absorption of UV radiation. The most evident utility of such glasses has been in control of visible light transmission. Inherently, however, they also strongly influence the intensity of UV transmission. This behavior is readily understood in terms of the Grotthus-Draper Law which states that: Only light that is absorbed can produce chemical change.
Photochromic glasses containing silver halide crystals absorb strongly at wavelengths shorter than 320 nm, but only weakly in the interval between 320 and 400 nm. Radiation in the wavelength range of 320-400 nm is much less harmful than that in the shorter wavelength region. Nevertheless, for some purposes, it would be desirable to eliminate transmission of this radiation as well. Therefore, it has been proposed to dope the above glasses with ions which provide additional absorption of UV radiation.
Photochromic glasses containing halides of copper and/or cadmium are also known, but not commercially available. Such glasses were originally disclosed in U.S. Pat. No. 3,325,299 (Araujo). The transmission cutoff in these glasses occurs at approximately 400 nm, and is much sharper than that in silver halide glasses. Consequently, protection against UV radiation is complete in these glasses without additional doping.
There are numerous applications for glasses having the sharp UV cutoff inherent in the copper or copper-cadmium halide glasses. Frequently, however, such applications require avoiding any change in visible absorption such as occurs in photochromic glasses exposed to UV radiation, e.g., sunlight. Therefore, it would be highly desirable to achieve the sharp UV cutoff characteristic of the copper and copper-cadmium halide glasses without the attendant photochromic behavior. It would also be highly desirable to produce such glasses that are essentially colorless because the yellow color associated with most UV absorbing materials is unacceptable for many applications. However, various fixed colors are desirable for other applications.
More recently, U.S. Pat. No. 5,322,819 herein incorporated by reference, disclosed a non-photochromic R.sub.2 O--B.sub.2 O.sub.3 --SiO.sub.2 glass which contains a precipitated cuprous or cuprous-cadmium halide crystal phase, and which has a sharp spectral cutoff at about 400 nm.
Two example application of this UV absorbing glass illustrate the problem.
Some LCD (liquid crystal display), projection systems use a straight line light path from a tungsten halide, xenon short arc or metal halide lamp through polarizers and LCD image screen followed by projection lenses. The complete UV as well as the IR radiation from these lamps must be blocked or the LCD screens and polarizers will be seriously degraded. The sharp UV cut-off absorbing glass could be used to block the UV radiation; however, there is the possibility for some long term darkening in the transmitted visible range from the shorter UV wavelengths produced by the 150 to 400 watt lamps.
Another potential application could be filters for laser pump lamps. UV absorbing filters are desired to block UV light emitted by 3 to 5 kW xenon pump lamps to protect organic optical adhesives in the light path to the lasing cavity, as well as reduce degradation of mirrors in the cavity. The most efficient filters will have high transmission maintained in the visible or near IR pump wavelengths. Again good UV absorption is desired without significant solarization.
In order to use the UV cut-off glass in filtering applications where the light source emits short wavelength UV it should be possible to protect the UV cut-off glass with other more solarization resistant glasses that cut-off the shorter wavelength UV light, allowing the UV cut-off glass to absorb out to the edge of the visible (.about.400 nm). This would permit good transmission in the visible range. Accordingly, it is the object of the present invention to provide strong UV blocking glass articles which are also resistant to solarization when exposed to intense UV radiation.